sysctl in a NSAutoreleasePool -> bus errors & segfaults

Question

Thanks in advance to everyone trying to help me. I'm having a big issue here, I've found some example code about sysctl and extended it so I can query the network interfaces for their in/out data. When I run this code directly in main() (without any NSAutoreleasePool) everything works fine. However, once I add it to my class and execute it I get bus errors & segfaults. I've tracked the issue down to NSAutoreleasePools. Can anybody please help? (If you don't believe it, just place a NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; before the code and place the whole thing into main())

int mib[] = { CTL_NET, PF_ROUTE, 0, 0, NET_RT_IFLIST, 0 };

    int alloc;

    struct if_msghdr    *ifm, *nextifm;
    struct sockaddr_dl  *sdl;
    char        *lim, *next;
    size_t      needed;
    char        s[32];

    char* buf;
    if (sysctl(mib, 6, NULL, &needed, NULL, 0) < 0)
        return 0;

    if (alloc < needed) {
        buf = malloc(needed);
        if (buf == NULL)
            return 0;
        alloc = needed;
    }

    if (sysctl(mib, 6, buf, &needed, NULL, 0) < 0)
        return 0;
    lim = buf + needed;

    next = buf;
    while (next < lim) {
        ifm = (struct if_msghdr *)next;

        if (ifm->ifm_type != RTM_IFINFO)
            return 0;
        next += ifm->ifm_msglen;

        while (next < lim) {
            nextifm = (struct if_msghdr *)next;
            if (nextifm->ifm_type != RTM_NEWADDR)
                break;
            next += nextifm->ifm_msglen;
        }        

        if (ifm != NULL && ifm->ifm_flags & IFF_UP) {
            sdl = (struct sockaddr_dl *)(ifm + 1);
            if (sdl->sdl_family != AF_LINK)
                continue;
            strncpy(s, sdl->sdl_data, sdl->sdl_nlen);
            s[sdl->sdl_nlen] = '\0';
            NSLog(@"interface %s in %qu out %qu \n", s,(UInt64)ifm->ifm_data.ifi_ibytes, (UInt64)ifm->ifm_data.ifi_obytes );
        }
    }

Answer 1

The NSAutoreleasePool isn't the problem. It just reveals that you have a problem with memory allocation, double releases, dangling pointers or uninitialized data.

In your case, it's uninitialized data. You don't initialize the alloc variable. As a consequence, it's completely random whether buf is allocated or is pointing to some random memory location.